A wire wound into a coil with overlapping layers (turns) insulated from one another functions as an inductive element and is commonly used in a current limiting application. Winding the coil around a material having little resistance to the flow of magnetic flux, i.e., a material which is easily magnetized, increases the inductance. Electrically conductive coils are frequently wound around a ferromagnetic core to increase the inductance. The inductance can be even further increased by using a "closed loop core," which is a core forming a ring or square or similar unbroken path. Alternatively, if a low inductance is desired, two coils may be wound in magnetic opposition on the same closed loop core, with the magnetic field of each coil cancelling the other. This produces a low impedance effect. When there is an imbalance in the currents between the two coils, the impedance increases. The capability to alter the impedance of the inductor by controlling the balance of the magnetic flux density forms the basis for use of the coil as a fault current limiter.
One approach to fault current limiting using a pair of magnetically coupled coils is disclosed in "Recovery Time of Superconducting Non-Inductive Reactor Type Fault Current Limiter," by T. Hoshino et al., Transactions on Magnetics, Vol. 32, No. 4, July 1996, which discloses the use of two superconducting coils with different critical currents non-inductively wound on a magnetic core in magnetic opposition. Under normal operating conditions, both coils are in the superconducting state and there is little resistance across the two coils. Current is shared equally between the two coils and there is no inductive voltage drop either across the coils. Under fault conditions one or both critical currents are exceeded to cause an imbalance in the currents in the coils and an increase in impedance for limiting the fault current. Because one of the coils must first become non-superconducting to provide the necessary resistance, restoration of normal operating conditions with removal of the fault may be delayed until the resistance in the coils decays to a low value and excessive heating may occur. Another approach to a superconducting fault current limiter is disclosed in "Tests of 100 kW High-T.sub.c Superconducting Fault Current Limiter," by W. Paul et al., IEEE Transactions on Applied Superconductivity, Vol. 5, No. 2, June 1995, which discloses an inductive superconductor fault current limiter where a superconductor shield prevents the formation of a field in the ferromagnetic core. Because this device is triggered magnetically and carries the total current load in the circuit, high currents in the circuit under normal conditions restrict the number of turns in the windings and limit performance under fault conditions. The current limiting performance of inductive fault current limiters based on Bi-2212 high temperature superconducting tubes is discussed in "Short Circuit Test Performance of Inductive High T.sub.c Superconducting Fault Current Limiters," by D. W. A. Willen et al., IEEE Transactions on Applied Super-conductivity, Vol. 5, No. 2, June 1995.
The present invention addresses the aforementioned limitations of the prior art by providing a passive fault current limiting device which presents a low impedance over a specified range of currents, converting passively and automatically to a large impedance when the current exceeds a predetermined design limit, and then passively and automatically returning to a low impedance value when the excessive current level drops below the design limit. The inventive fault current limiting device is capable of handling the electromechanical forces and thermal effects of large power components and is in the form of a small, easily installed and maintained package which can be economically manufactured.